Much Ado About Flapping

It bothered Jun Zhang, a physicist at New York University, that nobody could answer a question a child might ask at a parade: Why do flags flap in the wind? "Some people had believed that a flagpole causes the air to tumble, and the tumbling drives the flapping," he says, but nobody knew for sure. So he and his colleagues placed a silk thread in a dish of watered-down detergent and let the liquid flow through a channel, creating a two-dimensional model of a flag in the wind.

Photo by Jun Zhang/Applied Mathematics Laboratory/Courant Institute

At slow flow rates, the thread extended straight downstream with nary a wiggle. In a faster current, the thread began to undulate in a regular pattern (right). "We've shown that a flag doesn't need a pole to flap. The flapping is caused only by the interaction of the flag with the air," says Zhang. The source of the motion actually lies in the complicated interaction between moving fluid and a flexible body. Zhang hopes to use his two-dimensional simulation to analyze related phenomena, such as swimming fish, blood flow, and snoring. "Snoring may be due to the deformation of soft tissue in the throat. Our experiment could shed some light on what's going on."